Plio-Pleistocene volcanic activity in a stable terrain: Implications on magma generation and regional tectonics

English Abstract:

The presence of volcanism in an island arc setting is often attributed to an ubiquitous subduction. Palawan island, Philippines has been long been a subject of studies involving its origin, aseismic nature (e.g. Yumul et al., 2008), and absence of active, subduction-related volcanism. North of the island, however, a large basaltic lava field is observed. Geomorphologically young (from previous works indicating a Plio-Pleistocene age), the lava flow overlies Permian chert (Pena,2008). Relatively flat topography suggests low viscosity of flow. Preliminary petrography and X-ray Diffraction analyses of rock samples reveal presence of olivine, plagioclase and minor pyroxenes. Total Alkali versus Silica plot falls within basalt to basaltic-andesite field. K^­₂O vs. SiO­₂ - K^­₂O (from Peccerillo and Taylor, 1976), SiO2 –FeOt /MgO (from Miyashiro, 1974) and AFM ternary plots of samples fall within the Calc-Alkaline series. Spider plots (MORB-normalized, from Pearce, 1983) of basalt samples reveal unusual Nb-enrichment (~25ppm) not observed in typical island arcs (from previous data of Arcilla et al., 2003). Mechanism from Macpherson et al. in 2010 points to a possible small-degree partial melts of an OIB-like source, then interacting with high-degree partial melts from upper mantle (from Reagan and Gill,1989). This paper will restudy the occurrence of the enigmatic lava flow through additional petrographic approach supplementing previous information with recent geochemical data.